Solar Eclipse Yields Unique Paper Image

A daylong solargraph using photosensitive paper captured the entire solar arc interrupted by the moon passing in front of the sun, as seen August 21, 2017, near Lowman, Idaho.

After rising at the eastern horizon, the sun climbs toward its midday peak and then descends toward the west, with the gap depicting the depleted light of the eclipse. Mid-eclipse in Lowman was around 11:28 a.m. MDT. The blue image (top) on eclipse day using photosensitive paper is paired with a panorama of the same scene using a cell phone camera.

A solargraph is a pinhole camera that images the sun typically over several months, as several recent examples suggest. For the 2017 solar eclipse, I and some kids inserted blue Sunprint® paper into several aluminum cans with pinholes of varying sizes. We taped the cans to a tree facing southerly across the North Branch of the Payette River. Before the sun came up, I removed the tape in front of each hole to open the "shutter" and begin the exposure. After sunset, I recovered the cans.

Earlier I had proposed using Sunprint® paper to celebrate the eclipse as a daylong event instead of it being a mere snapshot in time. I'm awaiting results of another party that contacted me with their own plans for using the paper to capture totality.

Planetarium colleague John French introduced me to Sunprint® kits, which are sometimes sold in museum and planetarium gift shops. The Sunprint® website describes the process by which the paper works:

The areas of the paper exposed to the sun will fade from blue to white...Two crucial molecules in the paper are interacting, forming a new molecule. Their interaction is initiated by specific wavelengths of ultra-violet light. The new molecule is colorless so that as the blue molecules are converted, the white of the paper base begins to show through. Areas of the paper covered by your objects still contain the original blue molecule, so they remain blue.

The next step is to stop the "exposure" by soaking the paper in water for several minutes.

There are two exciting things happening underwater. First, the original blue compound is water soluble so that when you immerse it in the bath, the water carries it away, leaving only the white paper base in those areas. Second, the colorless compound whose formation was caused by the sun’s energy is not water soluble, so it cannot wash away in the water bath. It is sensitive to the water in another way. Just as the Sun’s light stimulated a chemical change in the previous step, the water stimulates another chemical change. The water causes an oxidation reaction that turns the colorless compound into the deep blue of a finished Sunprint.

I was joined in this experiment by Max W., Teddy W., Marlena W., and Avery D. We had mixed results because of the intentionally varying hole sizes and the directions the pinholes faced. The resulting paper images have to be turned right side up then horizontally flipped with software.

Thanks go to Richard Bergquist for providing the stunning riverside site from which we observed the 2017 solar eclipse and to the rest of the party who provided the empty 16-ounce beer cans. The skies were mostly clear, with a slight haze from distant forest fires. When the solargraph image is stacked on top of a panorama (taken with cell phone the next day) of the same scene, you can match the hills, nearby trees, and foreground river in the two images.